1. Technical Field
The present invention relates to the field of data transmission methods and apparatus and, more particularly, to such a method and apparatus involving a period shift keying of two or more predetermined frequencies, each representing a data value, wherein the number of transmitted cycles of each frequency per data value is predetermined.
2. Description of Related Art
Data transmission techniques are known which involve a keying of at least two predetermined frequencies wherein, at a receiver of a data transmission, data is recovered either from the phase (PSK) or frequency (FSK) of the transmitted frequencies. At a transmitter one bit value is represented by one frequency and the other bit value by a second frequency while at a receiver the data values are determined by either the frequency or phase of the received signal.
Implementations of either FSK or PSK data transmission involve a considerable investment in hardware for implementation comprising counters, registers, filters, discriminators, oscillators, dividers, comparators, amplifiers and delay circuits, each having particular purpose depending upon the implementation sought. For example, an oscillator output frequency may be divided by separate integers to the result that two predetermined frequencies may be provided which are keyed depending upon the incoming data stream. At a receiver, a discriminator is coupled to the output of filters for distinguishing the transmitted frequencies and hence the transmitted data values.
Such implementations overlook the opportunity to apply the internal capabilities of known microprocessors for serving as data transmitters and receivers, i.e., microprocessors may comprise many of the components found in data transmission and reception apparatus. Furthermore, it is becoming more and more common to include microprocessors in electronic apparatus generally which may require an interface to a data transmission medium. For example, the personal computer is but one example of electronic apparatus comprising a microprocessor which may be applied simultaneously for both self control and control of a data transmission interface with a telecommunications line. Besides personal computers other such electronic apparatus include cable television set-top terminals, alarm and energy management systems.
According to published Japanese patent application No. 58-114552 published Jul. 7, 1983, a frequency shift keying demodulation circuit has been constituted with a general-purpose microprocessor. The incoming signal is passed via a buffer amplifier, a low pass filter, an active bandpass filter, a limiter, and a comparator to a terminal of an input port of a microprocessor. It is the microprocessor which monitors the zero crossings of frequencies incoming to the terminal and, by comparison of the zero crossing time with a predetermined threshold value, determines the bit values of the transmitted data stream.
While such teachings demonstrate that efforts have been undertaken to efficiently utilize the attributes of known microprocessors, there still remains a requirement in the art to achieve yet further reductions in costs of hardware and to more efficiently utilize a microprocessor for, for example, data transmission as well as data reception.